1. Field of the Invention
The invention relates to particles having protected isocyanate groups on their surface, to a process for preparing them, and to coating formulations comprising said particles.
2. Description of the Related Art
Coating systems comprising particles—especially nanoparticles—are state of the art. Such coatings are described for example in EP 1 249 470 A2 or WO 03/16370. The particles in these coatings lead to an improvement in the properties of the coatings, particularly with regard to their scratch resistance and also their chemical resistance.
A frequently occurring problem associated with the use of the—generally inorganic—particles in organic coating systems consists in a usually inadequate compatibility between particle and coating-material matrix. This can lead to the particles being insufficiently dispersible in a coating-material matrix. Moreover, even well-dispersed particles may undergo settling in the course of prolonged standing or storage times, with the formation, possibly, of larger aggregates or agglomerates, which even on redispersion are then impossible or difficult to separate into the original particles. The processing of such inhomogeneous systems is extremely difficult in any case, and is often in fact impossible.
Favorable, therefore, is the use of particles which on their surface possess organic groups that lead to improved compatibility with the coating-material matrix. In this way the inorganic particle becomes “masked” by an organic shell. Particularly favorable coating-material properties can be achieved in this context if, furthermore, the organic functions on the particle surfaces also possess groups that are reactive toward the coating-material matrix, so that under the respective curing conditions of the coating material in question they are able to react with the matrix. In this way, success is achieved in incorporating the particles into the matrix chemically in the course of coating-material curing, which often may result in particularly good mechanical properties but may also result in improved chemical resistance. A system of this kind is described for example in EP 832 947 A.
Also known, furthermore, is the use of coatings comprising a binder which has been modified with nanoparticles. They can be prepared by reacting a particle equipped with a reactive functionality with a binder that carries a complementary function. Here, in other words, the organofunctional particle is incorporated chemically into the coating-material matrix not only on curing of the coating material but already at the binder preparation stage. Systems of this kind are described for example in EP 1 187 885 A or WO 01/05897.
In the case of one particularly important type of coating material, a film-forming resin is used which comprises hydroxy-functional polymers which, on curing of the coating material, are reacted with an isocyanate-functional curative. These polyurethane coating materials are notable for particularly good properties, such as a superior chemical resistance, but there is still a need for improvement, particularly as regards the scratch resistance of these systems. Typically they are used in particularly high-value and high-price fields of application: for example, as clearcoats or topcoats for OEM finishes in the automobile and vehicle industry. In addition, the majority of refinish coating materials for automobile repairs are composed of isocyanate-curing systems of this kind.
Typically a distinction is made between two different polyurethane coating systems, known as 2K and 1K systems. The former consist of two components, one of which is composed essentially of the isocyanate curative, while the film-forming resin with its isocyanate-reactive groups is contained in the second component. The two components must be stored and transported separately and should not be mixed until shortly before they are processed, since the completed mixture has only a greatly limited pot life. Often more favorable here are the so-called 1K systems, composed of just one component, in which alongside the film-forming resin there is a curative containing protected isocyanate groups. 1K coating materials are cured thermally, the protective groups of the isocyanate units being eliminated, with the deprotected isocyanates being able then to react with the film-forming resin. Typical baking temperatures of such 1K coating materials are situated at 130-150° C.
Particularly in the case of these high-value coating materials a further improvement in properties would be desirable. This is so in particular for vehicle finishes, both in the OEM and in the refinish segments. For instance, the achievable scratch resistance of conventional auto finishes, in particular, is still not sufficient, with the consequence, for example, that particles in the wash water in a car wash lead to significant marring of the finish. Over time, this causes lasting damage to the gloss of the finish. In this situation, formulations that allow higher scratch resistances to be achieved would be desirable.
One particularly advantageous way of achieving this objective is the use of particles having protected isocyanate functions on their surface. Where such particles are incorporated into 1K polyurethane coating materials, the isocyanate functions on the particle surfaces are liberated as well in the course of coating-material curing, and the particles are incorporated chemically into the finish. Moreover, the protected isocyanate functions, which are generally identical with the protected isocyanate functions of the coating-material curative, improve compatibility between particle and coating-material matrix.
Particles of this kind containing protected isocyanate functions are in principle already known. They are typically prepared by condensing a particle having free silicon hydroxide or metal hydroxide functions with an alkoxysilyl-functional organosilicon compound whose organic radical contains a protected isocyanate function. Organosilicon compounds of this kind containing masked isocyanate groups have already been described, in DE 34 24 534 A1, EP 0 212 058 B1, EP 872 500 A, JP 08-291186 or JP 10-067787, for example. A feature common to all of these compounds, however, is that between the masked isocyanate group and the silyl group there is a propylene group, as a result of which the hydrolytic reactivity and condensation reactivity of these silane compounds is low. Accordingly these compounds also possess low reactivity toward the silicon hydroxide or metal hydroxide functions of the particle, and so do not react at all or else react very slowly, or so that some of the NCO-protective groups start to cleave even under conditions suitable for the functionalization, elevated temperatures for example, as a result of which the particles can no longer be used in 1K polyurethane coating materials. This is true in particular of monoalkoxysilyl-functional silanes with protected isocyanate functions, whose reactivity is so low that they are usually completely unsuitable for the functionalization of particles. In certain cases, however, the use specifically of monofunctional alkoxysilanes would be particularly desirable, since using these silanes it would be possible with extreme ease to furnish the corresponding particles with protected isocyanate functions (readily accessible from a steric standpoint). Thus, monofunctional alkoxysilanes would possess the advantage that—subject to the proviso of a relatively high reactivity—they would react fully even without addition of water and in so doing would also not crosslink with one another to form a—usually relatively soft—shell around the particle.